[patch] libstdc++/61947 fix constructor ambiguity in std::tuple

[Jonathan Wakely]

This adds a tag parameter to the tuple base class constructors that
are used to implement uses-allocator construction but currently only
have a single parameter, ensuring they are not viable candidates when
calling a single-argument constructor.

Tested x86_64-linux, committed to trunk. I plan to fix this on the
release branches too at some point.
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